1. Field of the Invention
The present invention relates to an objective lens system used for an optical disk apparatus, an optical microscope, an image device, etc. It also relates to an optical disk apparatus incorporating such an objective lens system.
2. Description of the Related Art
One way to improve the data storing density in an optical disk is to make smaller the diameter of the beam spot produced on the recording region of the disk. In order to provide a small beam spot, use may be made of an objective lens system having a large numerical aperture (NA). However, as the NA is greater (more than 0.8 for example), the positional deviation of the lens relative to the rotating disk becomes less tolerable for achieving proper data-writing or data-reading operations. Thus, when the NA is great, it is necessary to adjust the orientation of the objective lens more accurately than when the NA is relatively small, so that the lens can follow the tilt of the disk properly.
A great NA may be achievable by combining more than one lens, as shown in FIG. 11. The illustrated objective lens system 90 includes two lenses (first and second lenses) 90A, 90B aligned vertically, to provide a great overall NA of the system. To address the tilting problem of the disk, the system 90 employs a floating slider design in which the second lens 90B is carried on a slider 91. The illustrated optical disk apparatus is categorized as a xe2x80x9csurface recording type,xe2x80x9d in which the slider 91 is held in facing relation to the recording layer (d) of the disk D. The slider 91 is provided with a coil 92 used for performing data-recording by magnetic field modulation. In the illustrated optical disk apparatus, a spherical lens is incorporated in the slider 91.
When the working distance is extremely short, the spherical lens may preferably be configured in a manner as shown in FIG. 12A or 12B. The spherical lens 90Bxe2x80x2 shown in FIG. 12A is of the so-called xe2x80x9chemisphere typexe2x80x9d, wherein the lens is formed into an exact half of a sphere. The lens 90Bxe2x80x2 has a hemispherical surface 90axe2x80x2 and a circular flat surface 90bxe2x80x2 upon which the center O of the sphere is located. The thickness d of the lens 90Bxe2x80x2 is equal to the radius of curvature of the hemispherical surface 90axe2x80x2. 
The spherical lens 90Bxe2x80x3 of FIG. 12B, on the other hand, is of the so-called xe2x80x9csupersphere typexe2x80x9d, wherein a flat cutting surface 90bxe2x80x3 is located below the center O of the sphere. The lens 90Bxe2x80x3 has a superspherical surface 90axe2x80x3 whose radius of curvature (r) is smaller than the thickness (d) of the lens 90Bxe2x80x3 (d=r+r/n, where xe2x80x9cnxe2x80x9d is the refractive index of the lens 90Bxe2x80x3).
In comparison with the lens 90Bxe2x80x3 of the supersphere type, the lens 90Bxe2x80x2 of the hemisphere type is advantageous in having greater tolerance for the thickness of the slider 91 and for the thickness of the lens 90Bxe2x80x2 itself. Also, the lens 90Bxe2x80x2 is advantageously used even when there occurs a rather great deviation from the prescribed working distance. This is because, in the hemisphere type, light beams strike upon the surface 90axe2x80x2 at right angles, the surface 90axe2x80x2 does not have a power, and the focal point fxe2x80x2 comes at the center O of the flat surface 90bxe2x80x2. In the hemisphere type lens 90Bxe2x80x2, however, the length of a light passage may significantly vary when the lens 90Bxe2x80x2 shifts in position. This is because the first lens 90A cooperating with the hemisphere type lens 90Bxe2x80x2 has a greater numerical aperture than when the supersphere type lens 90Bxe2x80x3 is used. Thus, the objective lens system 90 as a whole may fail to function properly when the two lenses 90A, 90Bxe2x80x2 are offset from each other even by a relatively slight amount. Accordingly, the tracking range may become unacceptably narrow. To compensate for this drawback, use may be made of an additional tracking mechanism. However, this scheme is disadvantageous since the disk apparatus incorporating such an additional unit is more expensive than otherwise.
When use is made of the supersphere type lens 90Bxe2x80x3, on the other hand, the objective lens system 90 composed of lenses 90A, 90Bxe2x80x3 is less vulnerable to the offset of the two lenses, whereby a wide tracking range is provided. This is due to the fact that the supersphere type lens 90Bxe2x80x3 has power, and the NA of the first lens 90A can be made small. However, since the lens 90B has power, the objective lens system is more vulnerable to the thickness variation of the slider 91 or spherical lens 90Bxe2x80x3. Also, the objective lens system may fail to function properly when there is a deviation from the prescribed working distance. Thus, to utilize the superspherical lens 90Bxe2x80x3, the tolerance of thickness for the slider 91 and the lens 90Bxe2x80x3 should be small. Unfavorably, this may lead to an increase in manufacturing cost.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide an objective lens system which can enjoy the advantages of both the hemisphere type and the supersphere type without suffering from their drawbacks.
Another object of the present invention is to provide an optical disk apparatus incorporating such an object lens system.
According to a first aspect of the present invention, there is provided an objective lens system that includes: a first lens provided with a first surface for allowing entry of light and with a second surface opposite to the first surface; and a second lens provided with a spherical third surface and with a flat fourth surface opposite to the third surface. The third surface faces the second surface and has a radius of curvature r0. The second lens has a thickness d0 and a refractive index n0. The thickness d0, the radius of curvature r0 and the refractive index n0 are determined so as to satisfy a relation: d0 greater than r0 greater than n0d0.
With such an arrangement, the second lens of the present invention can function as a hybrid that combines the conventional hemisphere and supersphere types. As a result, the second lens of the present invention can not only alleviate the shortcomings of the conventional hemisphere and supersphere lenses, but also enjoy the advantages of both of the lenses.
Preferably, the thickness d0, the radius of curvature r0 and the refractive index n0 may further satisfy the following relation: d0xe2x88x92r0 less than r0xe2x88x92n0d0.
According to a second aspect of the present invention, there is provided an optical disk apparatus that includes: a light source; and an objective lens system. The objective lens system includes at least one lens which is provided with a spherical surface having a radius of curvature r0. The lens has a thickness d0 and a refractive index n0. The thickness, the radius of curvature and the refractive index satisfy the following relation: d0 greater than r0 greater than n0d0.
According to a third aspect of the present invention, there is provided an optical disk apparatus that includes: a light source; an aspherical first lens which light emitted from the light source enters; a second lens that includes a spherical surface and a flat surface opposite to the spherical surface, the spherical surface facing the first lens and having a radius of curvature r0; and a storage disk facing the flat surface. The second lens is provided with both a spherical lens element including the spherical surface and a transparent base including the flat surface. The lens element has a thickness d1 and a refractive index n1, while the base has a thickness d2 and a refractive index n2. The radius of curvature r0, the thicknesses d1 and d2, and the refractive indexes n1 and n2 satisfy the following relation: d1+d2 greater than r greater than {n1/(n1+1)}d1+{n2/(n2+1)}d2.
Preferably, the refractive index n1 is greater than the refractive index n2 (n1 greater than n2).
Preferably, the refractive indexes n1 and n2 may further satisfy the following relation: 0 less than (n1xe2x88x92n2)/n2 less than 0.04.
Preferably, the second lens may serve as a slider designed to float on the storage disk by the air wedge effect when the storage disk is rotated at prescribed speed.
Preferably, the combination of the first and the second lenses may constitute a single objective lens system or unit which has a numerical aperture in a range of 0.8xcx9c0.95 for light having a wavelength of 405 nm(nanometers). Simultaneously, the first lens may have a numerical aperture in a range of 0.4xcx9c0.65 for the light of the same wavelength (i.e., 405 nm).
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.